1. Technical Field
The present invention relates to an antistatic tire. More specifically, the present invention relates to an antistatic tire of the type comprising electric current conducting elements incorporated in the tread and for grounding the static electricity of a vehicle.
2. Background Information
EP-A-718127 and EP-A-658452 relate to antistatic tires for grounding the static electricity of a vehicle, in which the tread and/or shoulders contain one or more electrically conducting elements defined laterally by a surface forming an annular portion of a rolling surface of the tread.
The tread and shoulders of a tire are normally made from different mixes, which react differently to the in-service stress to which they are subjected. That is, whereas the tread mix is designed to ensure a good dry and/or wet road-holding capacity, high resistance to wear and minimum rolling resistance, the. shoulder mix, which does not normally come into contact with the road surface, acts as an interface element between the tread mix and that of the sidewall (on the tire carcass), and therefore normally has the same physical and chemical characteristics as the sidewall mix.
In the event the shoulder mix comes into contact with the road surface, however, its physical and chemical characteristics must be compatible with those of both the tread and sidewall mixes.
In known antistatic tires, the above two mixes are combined with a third mix forming the electrically conducting elements, which, as stated, define a rolling surface portion of the tread.
The addition of a third mix, however, involves several drawbacks, due to the wear of the conducting mix normally differing from that of the mix from which the element incorporating the conducting mix is formed, thus resulting in uneven wear of the rolling surface of the tire, which in turn results in uneven wear of the tire, and in vibration and noise discernible from inside the passenger compartment of the vehicle.
Moreover, the use of three mixes, as provided for in the above documentsxe2x80x94a first mix for the tread, a second for the shoulder, and a third conducting mix-seriously complicates the normal tread extrusion process by requiring the use of a relatively complex, high-cost, three-way extruder.
It is an object of the present invention to provide an antistatic tire designed to overcome the aforementioned drawbacks.
According to the present invention, there is provided an antistatic tire comprising a tread, two sidewalls, and two annular shoulders, each interposed between the tread and a respective said sidewall; the tread and the sidewalls being formed respectively from a first and second mix; and the tread defining part of a rolling surface, at least an annular portion of which is electrically conducting and has an electrical resistivity of less than 109 ohmsxc3x97cm; characterized in that said electrically conducting portion of the rolling surface is defined by the shoulders, each of which is formed from a third electrically conducting mix, which, under low strain and in the cured state, has substantially the same rigidity as said first mix in the cured state, and, under high strain, has a greater rigidity than said first mix in the cured state.
Preferably, according to one embodiment of the above tire, said third electrically conducting mix, under low strain and in the cured state, has substantially the same mechanical characteristics as said first mix in the cured state, and, under high strain, has substantially the same mechanical characteristics as said second mix in the cured state.
Also according to the present invention, there is provided an antistatic tire comprising a tread, two sidewalls, two annular shoulders, each interposed between the tread and a respective said sidewall, and an electrically conducting element; the tread and the sidewalls being formed respectively from a first and second mix; and the tread defining part of a rolling surface, at least an annular portion of which is defined by an annular surface of said electrically conducting element, which has an electrical resistivity of less than 109 ohmsxc3x97cm, and is inserted inside the tread; characterized in that the conducting element and the shoulders are both formed from a third electrically conducting mix, which, under low strain and in the cured state, has substantially the same mechanical characteristics as said first mix in the cured state, and, under high strain, has substantially the same mechanical characteristics as said second mix in the cured state.
According to one embodiment of the above tire, said third mix, in the green state, preferably has substantially the same viscosity as said second mix in the green state.
Moreover, said third electrically conducting mix of the above tire preferably comprises: a polymer system or formulation similar to that of said first mix, so as to manifest a similar abrasion resistance; a system of high-structure carbon black-based reinforcing fillers; said structure being defined by over 100 ml/100 gr absorption of dibutyl phthalate, and the carbon black being in quantities of over 30 phr (parts per 100 of polymer); an antioxidation system similar to that of said second mix; plasticizers and activating and curing systems as in normal tire mixes, and in such quantities as to achieve said mechanical characteristics under high and low strain.